This invention relates to an automatic alignment correction technique and in particular, a technique for automatic beam alignment used in measuring, testing and verfying high contrast image patterns.
There exists in many systems a requirement for actual comparison of the average values of two or more signal levels. A typical example is in the field of electron beam (E-beam) mask inspection systems wherein alignment is obtained by scanning a pair of windows that straddle opposite edges of a feature to be inspected. By comparing back-scatter radiation detected during scanning of those windows, a position error signal is generated and alignment is ultimately obtained.
Reference is made to U.S. patent application Ser. No. 509,515, filed June 30, 1983 now U.S. Pat. No. 4,546,260 and entitled "Alignment Technique" wherein such a system is disclosed and relevant prior art is discussed. Other relevant prior art is found in U.S. Pat. Nos. 4,357,540 and 4,365,163. A common aspect of this prior art is the use of scanning windows which straddle opposite edges of a feature in an area to be inspected. Such are shown in FIG. 1 herein where two scanned windows are shown on opposite edges of a registration feature. In the case of the U.S. Pat. Nos. 4,365,163 and 4,357,540 an up-down integration alignment technique is employed. Such is shown in FIG. 2A. A proportioning circuit, shown in FIG. 2B, generates an alignment correction signal which is proportional to the inspection beam to inspected feature alignment error. While scanning the window straddling one side of a registration feature, the alignment circuit of FIG. 2B operates so that the input signal from a detector is integrated during the period T.sub.1 to T.sub.2 shown in FIG. 2A. While the window straddling the opposite side of the registration feature is scanned, the detector signal is inverted using the inverter 20 before being input to the integrator 22. These two components form a proportioning circuit.
The proportioning circuit thus integrates the signal obtained while scanning the first scanned window and modifies it by that obtained while scanning the second, during that is, the period T.sub.3 to T.sub.4, shown in FIG. 2A. The resulting correction signal C is then a measure of the magnitude and the direction of the overlay difference between the scanning beam and the target to be inspected.
As will be appreciated, the integration occuring during the first scan period T.sub.1 to T.sub.2 is modified by the integration occuring during the period T.sub.3 to T.sub.4. The resulting difference signal as processed in FIG. 2 constitutes the correction signal C which is held by the sample and hold circuit 24 during the time it must be used.
This technique employing an up-down integration technique to form the difference signal and the subsequent use of sample and hold has several disadvantages. The first is that the up and down ramps which are generated result from signal paths that involve a large number of different and variable components. Positive (up) and negative (down) integration is involved. This introduces an error in the resulting alignment correction signal that is a result of tolerances and the switching of components in the different signal paths.
Another difficulty is that the hold time of the sample and hold circuitry is not fixed but varies indefinitely with the number of inspection scans to be performed between updates. It will be appreciated that a sample and hold configuration is a compromise between the need to minimize droop for the longest probable hold time and the need for high speed operation. If the design criteria are such that reduction in droop for long periods is of paramount importance, then a large capacitance value must be used. This, in turn, increases the settling time of the sample and hold. Consequently, a design trade-off exists which places an upper bound on the time between updates.
Reference is also made to U.S. Pat. No. 3,811,069 which discloses an E-beam alignment technique that compares relative amplitudes of at least two frequency modulated signals. U.S. Pat. No. 3,919,550 relates to another analog technique for improving E-beam stability by providing focused and defocused signals, employing sample and hold techniques and feeding the sample and hold outputs to a differential amplifier. U.S. Pat. No. 3,937,959 relates to an E-beam autofocus circuit which generates DC voltages from two horizontal E-beam scans. These voltages are stored in analog memory circuits which are subsequently compared.
Thus, within the prior art, the use of analog circuitry is uniform and this introduces errors which in turn deteriorate the overall accuracy of the ultimate comparison required of such systems. Such is especially important in the context of E-beam alignment systems that require high degrees of accuracy.